1. Field of the Invention
The present invention relates to shaped charge tools for cutting pipe and tubing. More particularly, the invention is directed to methods and apparatus for improving the performance and cutting reliability of shaped charge tubing cutters.
2. Description of Related Art
The capacity to quickly, reliably and cleanly sever a joint of tubing or casing deeply within a wellbore is an essential maintenance and salvage operation in the petroleum drilling and exploration industry. Generally, the industry relies upon mechanical, chemical or pyrotechnic devices for such cutting. Among the available options, explosive shaped charge (SC) cutters are often the simplest, fastest and least expensive tools for cutting pipe in a well. The devices are typically conveyed into a well for detonation on a wireline or length of coiled tubing.
Although simple, fast and inexpensive, SC cutters are reputedly not the most reliable means for cutting tubing downhole. State-of-the-art SC cutters are typically tested and rated for cutting capacity at surface ambient conditions. In field use, however, downhole well conditions may exceed 10,000 psi and 400° F. The impact of such elevated pressur and temperature has upon SC performance, generally, is not well understood. High pressure/temperature test environments for SC tubing cutters is not a norm of th industry. Industrial standards for SC cutter performance provide only for cutting capacity at standard atmospheric conditions.
Physical testing under simulated well conditions has revealed two primary influence factors affecting the cutting capacity of SC cutters:                (1) The spacial clearance between the cutter perimeter and the inside wall of the tubing; and,        (2) Hydrostatic well pressure.        
Asymmetric alignment of the SC cutter within the flow bore of the tubular subject of a cut may reduce the SC cutting capacity up to 35% under atmospheric conditions. At 15,000 psi, SC cutting capacity is reduced an additional 20–25%.
The graph of FIG. 1 illustrates the performance of a typical, 1 11/16″ state-of-the-art SC tubing/casing cutter operating upon an L-80 grade, 4.7 lb./ft., 2⅜″ production tube. The abscissa axis of this graph plots the dimension of radial separation between the SC perimeter and the proximate tubing wall surface. When the SC cutter is aligned substantially coaxial with the tube, the clearance will be a uniform 0.15 in. around the SC perimeter as indicated by the dashed line coordinate that intersects the abscissa at the 0.15 in. value. The ordinate axis of the graph represents the wall penetration depth of an SC cutting jet. The dashed line coordinate from the ordinate axis represents the wall thickness of the tested tubing. The locus of curve “A” plots the SC performance at atmospheric pressure. The locus of curve “B” plots the SC performance at 15,000 psi.
To be noted from FIG. 1 is that even when the SC cutter is centrally aligned within the tube flow bore, the SC penetration capacity is marginal for completely severing the tube thickness at atmospheric pressure (curve A). When the pressure of the operational environment is raised to 15,000 psi, (curve B) the SC wall penetration capacity is substantially reduced. Similarly, when the SC is eccentrically misaligned with the tube axis wh reby one portion of the SC perimeter is in contact with the tube wall and the diametrically opposite portion of the SC perimeter has a 0.30 in. clearance, at atmospheric pressure the SC cutting capacity is reduced by 35%. Under 15,000 psi pressure, the cutting capacity is reduced by another 25% for a total of 60%.
Although SC cutter manufacturers offer centralizers for their tools and recommend their use, in field practice most cutters are operated without the use of a centralizer. However, such prior art centralizers are constructed of plastic or other low abrasion resistive material. Hence, such prior art centralizers are frequently damaged while running into a well by abrasion or by various restriction elements within the tubing bore. Consequently, a partial cut is the common result. As the data of FIG. 1 indicates, the penetration capacity of most cutters is marginal under optimum conditions and substantially lacking under severe conditions.
Another finding from test experiences is that SC cutters frequently lose penetrating capability when the cutter is mounted rigidly against the top sub of the tubing assembly or against the bottom of the SC cutter housing. The loss of cutting capacity is most severe when the SC is tightly coupled only on one side of the SC cutter. It would appear that the cutting jet generated by such a SC is asymmetrically formed due to such confinement. Such disruption of the normal jet formation also increases an undesirable flared distortion of the severed tubing wall at the separation plane and an undesirable deformation to the end face of the top sub.
In principle, the explosive assemblies of SC tubing cutters comprise a pair of truncated cones. The cones are formed as compressed powdered explosive material and joined along a common axis of revolution at a common apex truncation plane. The respective conical surfaces are faced or clad by a dense liner material; usually metallic. An aperture along the common conical axis accommodates a detonation booster.
In theory, ignition of the detonation booster initiates the SC explosive along the cone axis. Explosive detonation propagates a rapidly moving pressure wave radially from the axis through the two explosive material cones. Traveling radially from the con axis, the pressure wave first ncounters the charge lin r at the truncat d apex plane and progresses toward the conical base. As the two liners erupt from the conical surface into th proximate window space, heavy molecular material from the respective charge liners collide with substantially equal impulse along the common juncture plane. Since there is an included angle between the liners, the resulting vector of this collision is a substantially planar jet force issuing radially from the cone axis.
In sequence, the explosive material decomposes more rapidly than the liner material. Hence, the explosive material is transformed into a high pressure gaseous mass confined behind the liner barrier. I have discovered that if a portion of that gas escapes into the jet cavity between the conical liners in advance of the liner material merger, the intensity and direction of the cutting jet is compromised.
It is an object of the present invention, therefore, to provide the industry with tubing cutters having a substantially known downhole, high pressure cutting capacity.
Also an object of the present invention is to disclose a test method for quickly and inexpensively determining the cutting capacity of a cutter assembly under downhole conditions.
A further object of the invention is a cutter assembly design that reliably confines the decomposing SC explosive behind the SC liner to prevent distortion of the cutting jet development.
Another object of the invention is a reliable centralizer assembly.
Also an object of the invention is a new detonator booster design that ignites the SC booster substantially along the cone axis of the charges and at the common plane of apex truncation.
A further object of the invention is provision of an SC tube cutter explosive liner having deeper and more effective cutting capacity.